NOx emissions from vehicles with internal combustion engines are an environmental problem recognized worldwide. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. In conventional gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel powered vehicles and vehicles with lean-burn gasoline engines, however, the exhaust is too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel powered vehicles and lean-burn gasoline engines. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and homogenizing fuel-air mixtures can reduce NOx emissions. These techniques alone, however, will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, lean NOx traps (LNTs), and selective catalytic reduction (SCR).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiency with lean-burn catalysts is unacceptably low.
LNTs are NOx adsorbers combined with catalysts for NOx reduction. The adsorber removes NOx from lean exhaust. Periodically, the adsorber is regenerated by introducing a reducing environment. In the reducing environment, NOx desorbs and is reduced over the catalyst. The adsorbant is generally an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst can be a precious metal, such as Ru.
SCR involves the reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve NOx reductions in excess of 90%, however, there is concern over the lack of infrastructure for distributing ammonia or a suitable precursor. SCR also raises concerns relating to the possible release of ammonia into the environment.
U.S. Pat. No. 6,560,958 describes an LNT system in which hydrogen-rich synthesis gas (syn gas), including H2 and CO, is used as a reductant to regenerate the adsorber. The syn gas is produced from diesel fuel in a plasma converter. Periodically, the adsorber is taken offline from the exhaust system and supplied with the syn gas. A dual adsorber system is also described.
U.S. Pat. No. 6,732,507 describes a hybrid exhaust treatment system using an LNT and an SCR reactor in series. The SCR reactor captures ammonia produced by the LNT during regeneration and uses the captured ammonia to increase the extent of NOx conversion.
There continues to be a long felt need for reliable, affordable, and effective systems for removing NOx from the exhaust of diesel and lean-burn gasoline engines.